The red balls attached to high-voltage transmission lines are aerial markers to increase the visibility of power lines for low-flying aircraft. These early warning devices reduce the risk of airplanes and helicopters colliding with thin power lines that are barely seen from the air.
Aerial spherical markers are required to have a diameter of approximately less than 24 inches, as part of the safety flying regulations of the International Civil Aviation Organization, or ICAO. The standardized body also proposes an alternating color scheme when positioning the balls on power grid lines. This enhances conspicuousness of the markers against surrounding areas. Aside from red, aerial markers also come in aviation orange and white.